Method of developing an electrostatic image



Feb? 26, 1963 H. G. GREIG 3,079,272

METHOD OF DEVELOPING AN ELECTROSTATIC IMAGE Filed April 24. 1959 IEW- ' f/fmf Pmi/e INV EN TOR. /f/aw 0T Gef/6 United States Patent O 3,t79,272 METHD l? DEVELOPING AN ELECTRQ- STATlC MAGE Harold G. Greig, Princeton, NJ., assigner to Radio Corporation of America, a corporation of Delaware Filed Apr. 24, 1959, Ser. No. (2,438,568 6 Claims. (Cl. 117-37) This invention relates generally to electrostatic printing. More particularly, it relates to improved materials and methods for developing electrostatic images.

ln the art of electrostatic printing, electrostatic images are produced on the surface of an insulating material. Such images comprise a pattern of electrostatic charges on the surface. Visible images are commonly produced therefrom by cascading across the surface a dry mixture f finely-divided developer particles and substantially larger carrier particles. When the developer particles are triboelectrically-charged in the opposite polarity to the electrostatic charges they deposit in charged areas to produce a visible image in substantial configuration with the pattern of charges. When the developer particles have the same polarity as the electrostatic charges a visible image is produced in reverse configuration with respect to the pattern of charges.

"fhe foregoing method of developing electrostatic images is described in Elcctrofax Direct Electrophotographic irinting on Paper, by C. J. Young and H. G. Greig, RCA Review, Decem er 1954, vol. XV, No. 4. Also decribed in that publication are other methods of development such as: powder cloud, liquid mist and magnetic brush types.

The recording element may comprise almost any insulating surface but, preferably, the recording surface is also photoconductive to enable the recording ot light images. Recording elements comprising photoconductive selenium coated plates are described in US. Patent 2,297,- 691, issued October 6, 1942, to C. F. Carlson. Recording elements comprising photoconductive coatings on paper are described the Young and Greig publication, op. cit.

Recently, a so called liquid process tor developing electrostatic images has been proposed in which the solid developer particles are suspended in an insulating carrier liquid. Liquid development methods provide many distinct advantages over the use of dry developer mixtures and other methods of developing electrostatic images, for some applications. Basically the liquid developer previously described consists of timely-divided developer particles dispersed in a hydrocarbon liquid. This developer can be ilowed over a surface bearing an electrostatic image, or the surface can be immersed in a tray of liquid developer. lt can also be sprayed or rolled on to the surface. When appropriate developer particles are dispersed in a properly selected liquid, they acquire an electrophoretic or triboelectric charge enabling them to be attracted to an electrostatic charge pattern of appropriate polarity. Depositon of the developer particles on the charge image is an example of the phenomenon known as electrophoresis or cataphoresis. A liquid developer process for charge images is described in greater detail by l. A. Metcalf and R. l. lli/right in a paper entitled Xerography, published in the Journal of the Oil and Colour Chemists Association, November 1956, vol. 39, No. ll, London, England, and in another paper entitled Liquid Developers for Xerography published in the Journal of Scientic instruments, February 1955, vol. 32.

Although the above-mentioned liquid developer compositions are suitable for many purposes they do possess undesirable properties. Most hydrocarbon liquids are solvents for developer powders which include resins, waxes, or organic pigments. When resinous particles are dispersed in a hydrocarbon liquid they dissolve to some extent so that they become tacky and tend to agglomerate. rihus, dispersions must be freshly made a short time prior to use. If the dispersions stand for any extended period of time, the developer particles will ball up or cake. The tackiness of the developer particles caused by the hydrocarbon liquid can also make them adhere in unwanted image areas, which they may contact during development. Also, unless the image developed with such a dispersion is fixed in some manner, the tacky developer particles will tend to smear during handling. Fixing may be accomplished with a iixative spray or by heating. When hydrocarbon carrier liquids are employed, heating can be extremely dangerous in view of the lire hazard involved. ln addition to this, whether heated or not, most hydrocarbon liquids have an objectionable odor and the vapors thereof are generally toxic. For the foregoing reasons it can be readily seen that such liquids are unsuitable for many applications such as, for example, ofce copiers.

Accordingly, it is a general object of this invention to provide improved compositions of matter for developing electrostatic images.

It is a further object of this invention to provide an improved composition of matter, which composition is a solid until the time it is applied to an electrostatic image.

Another object of this invention is to provide an improved composition of matter for deve.oping an electrostatic image, which composition minimizes tire hazard.

lt is a further object of this invention to provide an improved composition of matter for developing electrostatic images wherein the image produced requires no xing.

it is yet another object of this invention to provide an improved composition of matter which when applied to an electrostatic image produces a visible image which is both Xed and glossed as a result of the developing step.

lt is yet another object of this invention to provide methods of developing electrostatic images which obviate the need for any tlxing or glossing steps.

The foregoing objects and other advantages are accomplished in accordance with this invention which provides an improved composition for developing electrostatic images, which composition is ordinarily a solid but which is heated and applied to the electrostatic image in liquid form to result in a tlxed developed visible image. The composition comprises a mixture of (l) a low-melting carrier material which upon heating becomes quite iluid and which is electrically-insulating in character; (2) a dispersed phase in the carrier material of a finely-divided developer substance. The low-melting material is selected to have a melting point of between about 50 C. and about 296 C. and a viscosity not in excess of about 400 centipoises at a temperature Within that range. The developer substance may comprise liquid droplets or solids but should be substantially insoluble in the carrier material and when dispersed therein must have an electrical character such that, when dispersed in the carrier material, it takes on an electrical charge.

Also contemplated in this invention is a method of developing electrostatic images employing the aforementioned composition. This method contemplates the steps of providing the above-mentioned developer composition, heating at least a portion of that composition to at least its melting point and contacting said melted portion across the electrostatic image to produce thereon a visible image of developer particles. Immediately upon conclusion of the development step, the image so produced is automatically fixed and in most cases can, if desired, present a gloss surface.

Specic examples and additional advantages of the developer compositions and of the improved methods of developing electrostatic images in accordance with this invention are included in the detailed description which follows and illustrated in the accompanying drawings wherein:

FIGURE 1 is a schematic, perspective view illustrating a developing method in accordance with this invention using a heated doctor blade;

FiGURE 2 is a schematic perspective view illustrating another developing method in accordance with this invention using a heated roller; and

FIGURE 3 is a schematic perspective view illustrating yet another developing method in accordance with this invention using a heated roller to develop an electrostatic image with developer composition picked up from a solid strip thereof near one edge of an insulating surface bearing the electrostatic image.

EXAMPLE I Paraiiin wax (melting point 55 C. to 79 C.) Carbon black Finely-divided carbon black is dispersed in the melt of the wax in proportions of up to 5 parts by weight of carbon black and 95 parts by weight of wax. The mixture is continuously stirred un-til a uniform dispersion is obtained and then allowed to cool to form a solid block of developer composition. Such a composition when remelted and applied to a surface bearing an electrostatic image will develop that image as a result of electrophoretic deposition of car-bon black particles in the charged areas of the image. This composition will, during development, provide a smooth glossy nish on the surface.

In Example I and in each of the examples to follow, the ratio of developer particles to carrier material in the developer composition may vary considerably. This ratio will depend to a large extent on the viscosity of the carrier material, the amount of charge taken on by the particles dispersed in the carrier material, particle size, and the density of the developer particle material. A lower limit, employing carbon black with about a .001 micron particle size, is about 0.05% carbon black and an upper limit about 5 Many low-melting materials may be substituted for the paraffin carrier specifically mentioned in Example I. These materials may be used either alone or in combination. Some suitable materials include:

(l) Carnauba wax, melting point about 84 C.

(2) Ultracera amber wax, melting point about 90 to 94 C. (a micro'crystalline petroleum wax of the Bareco l Oil Co., Barnsdall, Oklahoma).

(3) Flexo Wax C, non-crystalline hydrocarbon Wax of high adhesive properties, melting point between 62-64 C. (Glyco Products Company, Brooklyn, New York).

(4) Polymekon wax, melting point about 90 to 93 C. (a commercially modified microcrystalline wax, Warwick Wax Co., New York, N.Y.).

(5) B.E. Square wax white, melting point about 87 to 91 C. (a microcrystalliue petroleum wax, Bareco Oil Co.).

(6) Petranauba C. wax, melting point about 827 C. (a microcrystalline petroleum Wax, Bareco Oil Co.).

(7) Gum rosin, melting point about 100 to 140 C.

With any of the foregoing materials or combinations of materials, suitable additives such as plasticizers, elasticizers, toughening agents, and dispersing agents may be employed. Suitable additives may include:

1) Piccolyte S-135 (a thermoplastic hydrocarbon terpene resin, Pennsylvania Industrial Chemical Co., Clairton, Pa). When added in quantities, for example, of from 6% to 8% by weight to one of the foregoing developer compositions, the Piccolyte S-135 improvesv the durability of the developed image and substantiallyl lessens the waxy texture thereof.

(2) Aroclor (various mixtures of chlorinated biphenyls and polyphenyls, Monsanto Chemical Co., St. Louis, Mo). When added to the foregoing developery compositionsV in amounts, for examplel of from 6% to 8% by 4 weight, the Aroclor functions as a plasticizer, toughener, hardener, and improves surface texture of the developed image.

(3) Stearic acid, when added to the developer compositions in amounts, for example, of from 4% to 5% by weight, improves the triboelectric charge relationship lbetween the developer particles and the low-melt material o-f the composition to thereby facilitate the electrostatic attraction of the developer particles to an electrostatic image.

(4) Polyethylene (melting point about 115 C. to C.), when included in the developer composition in amounts, for example, of from 12% to 50% by weight, improvesuidity of the melted composition and provides a developed image having improved flexibility, and adhesion.

Thus, the foregoing and other additives or modifiers may be employed alone or in combination to alter` the electrical characteristics of the developer composition, or to alter the surface texture or flexibility of the developed image. With such developer compositions a developed image can be produced which has a high degree of flexibility, toughness and gloss; When such an image is produced on paper it will neither peel nor chip from the paper when tiexed.

Developer particles to be dispersed in the foregoing carrier materials may include any pigments or dyes or combinations thereof which are insoluble in the carrier material. Among these are the following:

(l) Powdered metals (2) Cyan Blue Toner GT (U.S. Patent 2,486,351 to R. H. Wiswall, Jr.)

(3) Benzidine Yellow (4) Rose Bengal (5) Hansa Yellow (Color Index No. 11680) (6) Pyrazolone pigments v (7) Nigrosne (8) Carbon Black Depending on particle size, these pigments may be dispersed in the carrier materials in'amounts of up to about 10% by weight of the composition, the smaller the particle size the less the amount of pigment (by weight) that is included in the composition.

All the foregoing developer compositions may be described as being direct printing for electrostatic images which comprise patterns ofl negative electrostatic charges. By this is meant that, when melted and applied to the Velectrostatic image, developer particles will be attracted to and deposit on those areas which bear negative electrostatic charges. Using these same materials it isY possible to produce reverse images. lThis may be accomplished by applying the developer composition to the electrostatic image with a metallic applicator and electrically biasing the applicator, and hence the developer composition, during the time it is applied tothe electrostatic image. When a negative bias is applied to such a composition, developer particles will deposit in the uncharged areas of the electrostatic image thereby producing a -visible image in reverse configuration with respect .to the electrostatic image. It is also possible to produce direct images of electrostatic images Vwhich comprises patterns of positive electrostatic charges. This may be done by employing a dcveloper composition in which the developer particles are triboelectrically negative. A specific example of such a composition is as follows:

EXAMPLE II "The carbonblack is dispersed in the polysiloxane and the dispersion ball *milled in a 2 'ounce glass jar with,

steel balls for about 40 hours. Employing this dispersion a mixture is prepared comprising:

3 grams carbon black dispersion 5 grams Vinylite VYNV (a copolymer of about 96% vinyl chloride, 4% vinyl acetate) 30 grams dimethyl polysiloxane Other resins which are predominantly polyvinyl chlo` ride, preferably 90% or more, may be used in place of the Vinylite VYNV. The foregoing mixture is again ball milled for about 16 to 40 hours. The reversal developer composition is then prepared by dispersing the above mixture into molten carrier material (any of those mentioned previously herein), the mixture constituting up to 20% by weight of the composition.

Color developer compositions may be prepared in a like manner and may comprise a mixture such as the following:

EXAMPLE III 11 to 14 grams Vinylite VNYV 2 grams pigment or dye grams dimethyl polysiloxane This composition is dispersed in a carrier material in a concentration of up to about 20% by weight of the composition. Specific examples of suitable pigments include:

(l) Pyrazolone pigment (Red) (Color Index No. 21080) (2) Hansa Yellow G (3) Patent Blue (Color index No. 672) The foregoing composition of Example li may also be employed to produce reverse visible images when the electrostatic image comprises a pattern of negative electrostatic charges. When the composition is applied to such an electrostatic image, developer particles will de posit on those areas of the image which bear no electrostatic charge.

Developing Methods The methods of this invention pro-vides many techniques for producing visible images from electrostatic images on an insulating surface.

The electrostatic image can be developed by contact with a heated surface carrying a thin film of molten developer composition. This heated surface may, as illustrated in FIG. 1 for example, comprise a doctor blade. The doctor blade may be biased to provide a reverse image or it may be biased to place ground close to the electrostatic image in which case it aids in filling in solid color areas. in lieu of a doctor blade, an electrostatic image may be developed by passing the surface on which it resides by and in contact with a heated roller carrying molt-en developer composition. The heated roller may again be biased to improve development or to provide a reversal print.

The developer composition may be applied in the form of a solid caire, to a surface bearing an electrostatic image. ln such a case the surface is heated to the melting point of the developer material and the solid caire is quickly passed over the surface to provide the developer image. in this instance, a developer composition having a melting point of about 60 C. or less is preferred in order that the electrostatic image not be dissipated by heat.

lt is also possible to provide a developer composition in the form of a solid cake to be employed in conjunction with a heated roller as illustrated in FIG. 2. A heated roller will readily pick up a film of melted developer composition from the solid cake by contact and, by rotating, transfer the molten composition to the surface on which the electrostatic image rests. The roller may be rolled across the surface or, in the alternative, may be caused to spin at a higher rate than that required for rolling. In the latter case, only the molten composition on the roller is in contact with the surface.

As illustrated in FIG. 2, another technique, particularly suitable for developing electrostatic images on photoconductive paper, contemplates employing a sheet of such paper which has a thin strip of solid developer composition along one edge of the photoconductive surface. Once the electrostatic image is produced on that surface, it is easily and quickly developed by contacting a hot roller to the strip of developer composition and rolling it across the surface.

Yet another technique comprises coating a member such as, for example, a sheet of paper or metal with the developer composition. This member is then superimposed on a surface bearing an electrostatic image, the coating on the member being in contact with the image bearing surface. The member is then heated to melt the developer composition whereupon a developed image is produced on the surface. Heating of the member may be conveniently accomplished by contacting a hot roller or other metallic member to the baci: of the member.

The methods of this invention also provide a unique technique for producing photographic transparencies. In such a case a thin transparent insulating sheet such as, for example, a sheet of one-half Mylar is superimposed on an insulating surface bearing an electrostatic image. Mylar is a polyester film; condensation product of ethylene glycol and terephthalic acid, E. l. du Pont de Nemours and Co., Wilmington, Delaware. `'vhen this is done, another electrostatic image (as a result of induction) will appear on the exposed surface of the insulating sheet. The image appearing on the insulating sheet can then be developed by any of the techniques described heretofore.

An unusual and surprising feature of this invention is the development of electrostatic images with developer compositions heated to temperatures of from 50 to 200 C. It is well known that elevated temperatures will dissipate electrostatic charges on any insulating surface. The dissipation of charges by heat is especially rapid with respect to electrostatic images produced on photoconductive surfaces. For example, the photoconductive coatings described in the Young and Greig publication, op. cit, can retain an electrostatic image at ordinary room temperatures for at least 50 minutes. The time during which such a coating can retain an image rapidly decreases with increasing temperature until at about 60 C. or above the charge storage time amounts to only a matter of a few seconds. Thus, employing temperatures ranging up to 200 C. as taught herein it would only be reasonable to expect that the electrostatic image would be dissipated so rapidly as to obviate development thereof. It is not well understood how such images can be developed at temperatures such as, for example, C. It can only be conjectured that, although the developer composition is at this temperature, its contact with the photoconductive surface does not result in raising the temperature of that surface to too high a level or for a long enough time to prevent deposition of developer particles from the molten composition.

What is claimed is:

l. A method of developing an electrostatic image on an insulating surface comprising the steps of: providing a solid body of developer material comprising finely-divided electroscopic developer particles dispersed in an electrically-insulating thermoplastic carrier material having a melting point substantially within a range of from 50 C. to 200 C.; heating said solid body to melt at least a portion thereof; and contacting said molten portion across said insulating surface to produce thereon a visible image of said developer particles.

2. The method of claim l wherein said portion of said body is melted by contacting thereto a heated roller and wherein said visible image is produced by contacting said surface with said molten portion carried on the surface of said roller while rotating said roller.

3. The method of claim 1 wherein said portion of said .body is melted in contact Vwith a heated doctor blade and -caused to flow toward one edge thereof and wherein said visible image is produced by contacting said edge with said molten portion thereon across said surface.

4. A.method 0f developing an electrostatic image on a photoconductive surface comprising the steps of providing a solid body of developer material comprising nelydivided electroscopic developer particles dispersed in an electrically-insulating thermoplastic carrier material having a melting point within a range of from 50 C. to 70 C.; heating said surface to a temperature Withinsaid range and simultaneously and rapidly contacting said solid body across said surface thereby melting a portion of said solid body and producing on said surface a visible image of said developer particles.

5. A method of developing an electrostatic image on an insulating surface comprising the steps of: providing a member having a surface coated with a solid composition of developer material comprising finely-divided electroscopic developer particles dispersed in an electricallyinsulating thermoplastic carrier material having a melting point substantially within a range of from 50 C. t0 200 C., contacting said coated surface to said insulating surface and, while so contacting, applying heat to said coating to melt said thermoplastic material vto produce on said insulating surface a visible image of said developer particles.

6. A method of electrostatic printing comprising the steps of: providing an insulating surface having along one edge thereof a solidV strip ofA developer material having a melting point substantially within a range of from 50 C. to 200 C., comprising 'finely-divided developer particles dispersed in an electrically-insulating thermoplastic carrier material; producing an electrostatic image on another portion of said surface and applying a roller heated to a temperature Within said range to said strip of developer material and then rolling said roller across saidV surface to develop said electrostatic, image.

References Cited in the le of this patent UNITED STATES PATENTS OTHER REFERENCES Zimmerman et al.: Handbook of Material Trade Names, 1951, page 606.

Seymour: Hot Organic Coatings, 1959, pp. 52 to 56. 

